1. Field of the Invention
The present invention relates to a high voltage over-current protection device, and more particularly to a high voltage over-current protection device with a PTC behavior.
2. Description of the Prior Art
The resistance of conventional PTC devices is sensitive to changes in temperature. When a PTC device is operated normally, the resistance remains at an extremely low value, so that the circuit operates normally. When the temperature rises to a critical value due to the occurrence of an over-current or over-temperature situation, the resistance of the PTC device may jump instantly to a high resistance state (for example, over 104 ohm) to impede the excessive current, thereby protecting cells or circuit elements. Because the PTC device can effectively protect electronic products, the PTC device has been integrated into various devices to prevent damage caused by over-current.
Conventional PTC over-current protection devices used in high-voltage (over 250 volts) applications usually have a hot line layer or a hot zone in the PTC material layer when being tripped. The hot line layer is caused by the heat generated as the PTC material layer withstands most of the voltage. Moreover, compared with other regions of the PTC material layer, the hot line layer has a higher resistance. When current flows through the PTC material layer, the hot line layer is heated rapidly. When the temperature of the hot line layer rises (the resistance value rises at the same time), even if the current flowing through the PTC material layer is decreased, the increased resistance of the hot line layer will cause a rapid heating rate of the hot line layer, and a degradation of the polymers occurs in the hot line layer, thus resulting in the loss of the high voltage withstanding characteristic of the over-current protection device and damage to the over-current protection device.
Under high-voltage tripped state, the temperature at the hot line layer is much higher than the temperature at other area. This extremely non-uniform temperature distribution causes local non-uniform voltage withstanding property which results in local voltage breakdown failure. The high voltage withstanding capability of the PTC device depends strongly on the temperature dissipation capability. Good thermal management is essential to the high voltage withstanding characteristics of the PTC device.
Further, as for the fabricating process of an over-current protection device for high-voltage applications, U.S. Pat. Nos. 5,227,946 and 5,195,013 disclose a PTC over-current protection device, wherein the included polymers are irradiated to enhance their physical and electrical properties. Thereby, the high voltage withstanding characteristics of PTC over-current protection devices can be improved. However, the polymers may be degraded by the irradiation, and larger molecules are broken down into small molecules, thus losing the original physical and electrical properties. Moreover, the irradiation on the PTC material layer often is not uniform, which may deteriorate the ability to withstand high voltage. In addition, if the Co-60 γ-ray irradiation process is used for crosslinking, the irradiation must take a large amount of time to reach the required high irradiation dosage since the irradiation energy of Co-60 γ-ray is low. Consequently, the production throughput is greatly reduced. If the E-beam irradiation process is used for crosslinking, the irradiation time can be drastically reduced. However, the internal stress inside the PTC matrix may be incurred as large amount of heat is generated during the irradiation. The internal stress could result in deterioration of the PTC voltage endurance. The rapid generation and slow dissipation of heat during the irradiation process make the fabricating process difficult to control. The variation of temperature during the fabricating process causes inconsistent product quality as well as deteriorated PTC performance. Consequently, the high yield loss from the fabricating process results in high cost of the PTC device.